记平行四边形ABCD的面积为s, AB长度为a

用面积法来计算CR的长度

[amp]=1/2[apb]=1/2 * 3/8 * [abc] = 1/2 * 3/8 * 1/2 * s = 3/32 * s
[apr]=[amr] - [amp] = 1/4 *s - 3/32 * s = 5/32 * s
[prc]=5/3[apr]= 25/96 * s
[apc]=[apr] + [prc] = 5/32 * s + 25/96 * s = 5/12 * s
[adc]=1/2 * s
CR:CD=[arc]:[adc]=(5/13 * s) : (1/2 *s) = 5:6
CR=5/6 * CD= 5/6 * a